1. Field of the Invention
The present invention relates to a color liquid crystal display apparatus having a color saturation correcting circuit for red, green, and blue.
2. Description of the Prior Art
Recently, color liquid crystal television sets and color liquid crystal displays have been put into practice and widely used.
FIG. 1 is a block diagram showing a circuit arrangement of a conventional color liquid crystal television set.
Referring to FIG. 1, reference numeral 100 denotes a liquid crystal panel on which an image is displayed. The liquid crystal panel 100 is designed such that a liquid crystal is sealed between a substrate having signal electrodes 100a and a substrate having scanning electrodes 100b. The liquid crystal panel 100 further comprises color filters for color display. A matrix type panel is exemplified as liquid crystal panel 100, in which pixels are formed by cross portions between the signal and scanning electrodes 100a and 100b.
A flow of signals until an image is displayed on the liquid crystal panel 100 will be described below.
Reference numeral 1 denotes an antenna. When an external radio wave is received by the antenna, a video signal SS is formed by a receiver 2. The video signal SS is supplied to a color separation circuit 3, and is separated into a red (R) video output signal RS, a green (G) video output signal GS, and a blue (B) video output signal BS. The R, G, and B video output signals RS, GS, and BS are respectively A/D-converted by A/D converters 4, 5, and 6, and are output as 4-bit signals DR, DG, and DB. The reference potentials of each A/D converter are set to high and low reference potentials V.sub.H and V.sub.L. The 4-bit signals DR, DG, and DB are arrayed by a multiplexer 7 in accordance with the arrangement of the color filters, and a 4-bit signal DP is output. The 4-bit signal DP is stored in a shift register 8. When data corresponding to one scanning operation is stored in the shift register 8, the stored data is supplied to a pulse width modulating circuit 9. The data which has been subjected to pulse width modulation in the pulse width modulating circuit 9 is supplied to the signal electrodes 100a of the liquid crystal panel 100.
The video signal SS is also supplied to a control circuit 10. The control circuit 10 generates clocks CKR, CKG, and CKB used in the A/D converters 4, 5, and 6, a clock CK1 used in the multiplexer 7, a clock CK2 used in the shift register 8, a clock CK3 used in the pulse width modulating circuit 9, and clock TPR to be supplied to a timing pulse generator 11. An output from the timing pulse generator 11 is received by the scanning electrodes 100b of the liquid crystal panel 100.
FIGS. 2(a), 2(b), and 2(c) respectively show relationships between the above-described R, G, and B video output signals RS, GS, and GB, and the high and low reference potentials V.sub.H and V.sub.L. A time and a voltage are plotted along the axis of abscissa and the axis of the ordinate, respectively.
Reference symbol HS denotes a horizontal sync signal; V.sub.Q, a pedestal level serving as a reference for each video output signal and representing the minimum value of each signal associated with gradation display; and V.sub.P, the maximum value of each video output signal. V.sub.Q &lt;V.sub.L, and V.sub.H &lt;V.sub.P are established for each video output signal.
FIGS. 3(a), 3(b), and 3(c) respectively show voltage drive waveforms to be applied to the pixels formed on the first scanning electrode. FIG. 3(a) shows a voltage waveform when an ON voltage V.sub.ON is applied to the pixels. FIG. 3(c) shows a voltage waveform when an OFF voltage V.sub.OFF is applied to the pixels.
FIG. 3(b) shows a voltage waveform when a root-mean-square (RMS) voltage between the ON and OFF voltages V.sub.ON and V.sub.OFF is applied to the pixels. Reference symbol t.sub.A denotes a time period corresponding to one field; t.sub.B, a selection period; t.sub.C, a non-selection period; and t.sub.D, a blanking period. Reference symbol V.sub.O denotes a voltage amplitude in the selection period; and b, a bias ratio obtained from the conditions for the maximum ratio of the ON voltage V.sub.ON to the OFF voltage V.sub.OFF.
FIG. 4 shows a change in transmittance T with respect to an RMS voltage V.sub.rms applied to the liquid crystal panel at a certain viewing angle. The axis of abscissa represents the RMS voltage V.sub.rms applied to the liquid crystal. The axis of ordinate represents a relative transmittance, assuming that a state wherein no light is transmitted is set to be a reference, and a state wherein a transmittance T is saturated upon increasing an applied voltage is set to be 100%. In FIG. 4, curves R and GB represent transmittance curves of R, and G and B pixels. The three transmittance curves R, G, and B do not necessarily coincide with each other. For example, a certain color pixel may be deviated from the other two color transmittance curves. FIG. 4 shows a case wherein the transmittance curve of an R pixel is different from the transmittance curves of G and B pixels. In the liquid crystal television set, a bright volume is arranged to change a drive voltage amplitude while the ratio of the ON voltage V.sub.ON to the OFF voltage V.sub.OFF is set substantially constant so as to obtain a clear display. When the bright volume is set to a position where an image can be displayed, the OFF voltage V.sub.OFF for a display at the minimum transmittance is determined. In a liquid crystal display apparatus in which a time-divisional driving is performed, if the number of divisions is set to be N, the following relationship is obtained from the conditions for setting the maximum ratio of the ON voltage V.sub.ON to the OFF voltage V.sub.OFF : ##EQU1## Therefore, if the OFF voltage V.sub.OFF is determined, the ON voltage V.sub.ON is automatically determined. In FIG. 4, T.sub.OFFR represents the transmittance of an R pixel and T.sub.OFFGB represents the transmittance of G and B pixels when the OFF voltage V.sub.OFF is applied. In addition, T.sub.ONR and T.sub.ONGB respectively represent the transmittance of the R pixel, and the G and B pixels when the ON voltage V.sub.ON is applied.
As described above, in order to apply the RMS voltage V.sub.OFF in FIG. 4 to the pixels, the apparatus is required to be driven as shown in FIG. 3(c). The drive waveform in FIG. 3(c) is converted from a signal level V.ltoreq.VL in FIGS. 2(a), 2(b), and 2(c). In order to apply the RMS voltage V.sub.ON in FIG. 4 to the pixels, the apparatus is required to be driven as shown in FIG. 3(a). The drive waveform in FIG. 3(a) is converted from a signal level V.gtoreq.VH in FIGS. 2(a) and 2(b).
In the conventional color liquid crystal television set, however, the minimum transmittances of an R pixel, and G and B pixels are determined by the OFF voltage V.sub.OFF as shown in FIG. 4, and the transmittances at this time are respectively set to be T.sub.OFFR and T.sub.OFFGB.
In addition, the maximum transmittances of the R pixel, and the G and B pixels are determined by the ON voltage V.sub.ON, and the transmittances at this time are respectively set to be T.sub.ONR and T.sub.ONGB. If a difference between T.sub.OFFR and T.sub.OFFGB, or between T.sub.ONR and T.sub.ONGB is increased, the saturation of R and B will be different from each other and the saturation of G and B will also be different from each other. This poses a problem that a natural image cannot be obtained. More specifically, if the reference potentials for the liquid crystal panel in FIG. 4 are set as shown in FIG. 2, the overall display image is tinged with red. This phenomenon becomes most noticeable when an image is displayed upon reception of a white raster signal. This is caused mainly by a difference between T.sub.ONR and T.sub.ONGB.
In FIG. 4, the transmittance curves of the G and B pixels exhibit the same characteristic. However, even if the transmittance curves of the R, G, and B pixels exhibit different characteristics, a similar problem is posed.
In mass production, since the thickness and spectral characteristics of color filters interposed between signal and scanning electrodes vary, the transmittance curves of R, G, and B may differ from the respective preset values, and hence are required to be electrically corrected.